Cyclobutanone is a strained cyclic motif that is present in many natural products, bioactive molecules, and materials. Benzocyclobutenones (BCBs), which possess higher ring strain than saturated cyclobutanones due to their rigid planar architectures, are versatile building blocks for the transition-metal-mediated C-C bond activation to access diverse skeletons that are nontrivial to synthesize otherwise^1,-3. Although many efforts have been devoted to the synthesis of BCB skeletons in the past few decades, the construction of cyclobutanones that are fused to nonbenzene aromatics remains an uncharted area in synthetic chemistry.

Fig. 1 | Catalytic cycloaddition & cyclization methods. a Catalytic carbocyclization & carbene/alkyne metathesis (CAM). b This study: catalytic 4-exo-dig carbocyclization process for the direct construction of furan-fused cyclobutanones vs competitive 5-endo-dig cyclization.

In the past two decades, transition-metal complex catalyzed alkyne carbocyclization has emerged as a powerful method in organic synthesis for the direct construction of carbocyclic and heterocyclic structures from different functionalized alkyne precursors. In this context, the carbene/alkyne metathesis (CAM)^4,5 cascade reaction is one of the elegant cyclization protocols for the straightforward preparation of 5- and 6-membered carbocyclic structures through different types of exo- or endo-dig cyclizations (Fig. 1a, path e-h). However, the 4-exo-dig cyclization process is certainly the least-explored one, and no successful example has been disclosed thus far (Fig. 1a, path i), which might be due to the competitive 5-endo-dig pathway that was proposed to be more sterically favorable.

As continuing our interests in catalytic alkyne functionalization^6-8, we were curious about the feasibility of enabling an uncharted catalytic 4-exo-dig cyclization process for the direct construction of four-membered structures. To the best of our knowledge, the catalytic 4-exo-dig carbocyclization of alkynes for the construction of cyclobutanones has not been reported. Herein, we present our recent results in this direction, a Rh-catalyzed formal [3+2] annulation of diazo group tethered alkynes involving a 4-exo-dig carbocyclization process. This methodology provides direct and convenient access to the synthesis of furo[3,4-b]cyclobutanones via an intramolecular annulation of the in-situ formed zwitterionic intermediates rather than through a CAM process involving vinyl carbene species (Fig. 1b, path j). In our protocol, a vinylic sp²-hybridized carbon has been introduced as the key linkage of the diazo group and the triple bond, instead of using a flexible aliphatic chain, which could facilitate the 4-exo-dig carbocyclization by bringing the reacting centers in close proximity. Thus, one of the serious drawbacks, the inherent propensity of the competitive 5-endo-dig carbocyclization of alkynes towards thermodynamically stable tricyclic products through the endocyclic carbene intermediates, could be overcome (path k). Moreover, density functional theory (DFT) computation elucidates that the favorable 4-exo-dig cyclization is mainly attributed to the relatively lower angle strain of the key sp-hybridized vinyl cationic transition state in the cyclization step. The synthesized cyclobutanone-fused furan derivatives with imbedded furyl, carbonyl, and benzylidene motifs have not been previously reported and are potential versatile synthetic building blocks. This practical method could be successfully applied for the late-stage modification of complex natural products and pharmaceutical molecules, and the synthetic application of these unique compounds has been demonstrated through a variety of ring-opening reactions and cycloadditions for the diversity-oriented-synthesis. Continuing works on the exploration of novel catalytic transformations with this intriguing furan-fused cyclobutanone as a 4C synthon in cycloaddition reactions, including the asymmetric catalytic version, are going on in the lab and will be reported soon.

References

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